Thermoelectric devices are notable in that they have no moving parts. Solid state thermoelectrical conversion relies only on a temperature differential to generate a current or, conversely, an electrical current to generate a temperature differential, i.e. to heat or cool. Thermoelectric devices have been used extensively since the early 1960's as reliable power supplies for remote applications such as spacecraft for deep space exploration. Specific examples are radioisotope thermoelectric generators (RTG) which use radioisotopes as heat sources. Galileo, Cassini, and other spacecraft are powered by reliable RTG units.
Efficient thermoelectric materials are semiconductors such as silicon, germanium, silicon-germanium alloys, lead telluride, bismuth telluride, and bismuth selenide. The thermoelectric efficiency of power conversion is indicated by the dimensionless figure of merit ZT, which is given by the following equation:
  ZT  =                    S        2            ⁢      σ      ⁢                          ⁢      T        K  where S, σ, K and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively. Until recently, ZT values for all thermoelectric materials have been less than 1, thereby limiting their use to applications, such as space vehicles, where low conversion efficiency is tolerated due to the necessity for such specialized applications. One thermoelectric material of choice for RTG applications is silicon germanium alloys with a ZT between 0.5 and 0.8.
Recently, microstructural engineering has resulted in thermoelectric materials exhibiting ZT values approaching and even exceeding unity. One successful approach involves the creation of ultrafine (nano) powder composites of thermoelectric materials. It is postulated that the high density of physical interfaces (e.g. grain boundaries) in the nano composites acts to impede phonon propagation resulting in decreased thermal conductivity while maintaining electrical conductivity thereby enhancing S2 and ZT, the thermoelectric figure of merit.